These days, it is predominantly microlithographic projection exposure methods that are used for producing semiconductor components and other finely structured components, such as e.g. photolithography masks. Here, use is made of masks (reticles) or other pattern generating devices, which carry or form the pattern of a structure to be imaged, e.g. a line pattern of a layer of a semiconductor component. The pattern is positioned in the region of the object plane of the projection lens between an illumination system and a projection lens in a projection exposure apparatus and it is illuminated by illumination radiation provided by the illumination system. The radiation modified by the pattern travels through the projection lens as projection radiation, the projection lens imaging the pattern with a reduced scale onto the substrate to be exposed. The surface of the substrate is arranged in the image plane of the projection lens optically conjugate to the object plane. The substrate is generally coated by a radiation-sensitive layer (resist, photoresist).
It is desirable to generate structures with increasingly smaller dimensions on the substrate by way of lithography. In the case of e.g. semiconductor components, smaller structures can lead to higher integration densities; this generally can have an expedient effect on the capability of the microstructured components produced.
The size of the generable structures depends decisively on the resolving power of the employed projection lens and can be increased, firstly, by reducing the wavelength of the projection radiation used for the projection and, secondly, by increasing the image-side numerical aperture NA of the projection lens used in the process.
These days, highly resolving projection lenses operate at wavelengths of less than 260 nm in the deep ultraviolet (DUV) range or in the extreme ultraviolet (EUV) range.
In order to ensure a sufficient correction of aberrations (e.g. chromatic aberrations, image field curvature) in the case of wavelengths in the deep ultraviolet (DUV) range, use is usually made of catadioptric projection lenses which have both transparent refractive optical elements with refractive power (lens elements) and reflective elements with refractive power, i.e. curved mirrors. Typically, at least one concave mirror is contained. In this case, a resolving power enabling a projection of structures with dimensions of 40 nm is obtained these days with immersion lithography at NA=1.35 and λ=193 nm.
Integrated circuits are produced by a sequence of photolithographic structuring steps (exposures) and subsequent processing steps, such as etching and doping, of the substrate. The individual exposures are usually performed using different masks or different patterns. So that the completed circuit exhibits the desired function, it is desirable for the individual photolithographic exposure steps to be matched to one another to the best possible extent such that the manufactured structures, e.g. contacts, lines and the components of diodes, transistors and other electrically functional units, come as close as possible to the ideal of the planned circuit layout.
Manufacturing errors may arise, inter alia, when the structures generated in successive exposure steps do not lie sufficiently closely on one another, i.e. if the overlay accuracy is insufficient. The overlay accuracy of structures from different manufacturing steps of a photolithographic process is usually referred to by the term “overlay”. This term denotes, for example, the overlay accuracy of two successive lithographic planes. The overlay is an important parameter when manufacturing integrated circuits because alignment errors of any type can cause manufacturing errors, such as short circuits or missing connections, and thus restrict the functionality of the circuit.
High demands are also placed on the overlay accuracy of successive exposures in multiple exposure methods. By way of example, a substrate, for example a semiconductor wafer, is exposed twice in succession in the double patterning method (or double exposure method) and the photoresist is processed further thereafter. By way of example, a normal structure with a suitable structure width is projected in a first exposure process. A second mask with a different mask structure is used for a second exposure process. By way of example, periodic structures of the second mask can be displaced by half a period in relation to periodic structures of the first mask. In the general case, the differences between the layouts of the two masks can be large, particularly in the case of more complex structures. Double patterning can achieve a reduction in the period of periodic structures on the substrate. This can only succeed if the overlay accuracy of the successive exposures is sufficiently good; i.e., if the overlay errors do not exceed a critical value.
An insufficient overlay can therefore significantly reduce the yield of good parts during the manufacturing, as a result of which the manufacturing costs per good part increase. WO 2014/139719 A1 describes a projection lens with a wavefront manipulation system for controllable influence of the wavefront of the projection radiation travelling from the object plane to the image plane of the projection lens. The wavefront manipulation system has a manipulator having a manipulator surface arranged in the projection beam path. It is arranged in a near-field manner, i.e. at a small distance from the closest field plane, for example between the object plane and a first subsequent lens. The manipulator includes an actuating device which renders it possible to modify the surface form and/or the refractive index distribution of the manipulator surface in a reversible manner. The manipulator is configured in such a way that a plurality of maxima and a plurality of minima of an optical path length change of the projection radiation can be generated in accordance with a characteristic period over an optically used region of the manipulator surface. As a result of this, it is possible, inter alia, to keep overlay errors small or to reduce the latter.